Field of the Invention
The present invention relates to an electric power steering apparatus in which a power of an electric motor works directly on a steering system as a steering assist power to thereby reducing the steering power of a driver, and more particularly to an electric power steering apparatus for automatically compensating a torque reference value of a torque sensor.
A conventional electric power steering apparatus is known in which a target current is set on the basis of a steering torque detected by a steering torque sensor, an electric motor is driven with an electric motor voltage in correspondence with this target current, the electric motor current that flows through the electric motor is detected, a negative feedback (NFB) is applied to the electric motor, the electric motor voltage is controlled on the basis of a deviation signal of the target current and the electric motor current so that the electric motor is driven with the electric motor current that is equal to the target current, and the steering assist power works on the steering system.
FIG. 6 is a structural diagram showing this kind of conventional electric power steering apparatus.
FIG. 6, the electric power steering apparatus is provided with a steering detecting means (steering torque sensor) 1, a target current setting means 2 for setting a target current Im using the characteristic relationship between the steering torque T and the target current Im shown in FIG. 7 on the basis of the steering torque T from the steering torque detecting means 1, an electric motor current detecting means 4 for detecting the current that flows through an electric motor 3, a deviation determining means 5 for calculating a deviation signal xcex94I between the target current Im and the electric motor current I detected by the electric motor current detecting means 4, and a drive controlling means for converting the deviation signal xcex94I into the voltage and for PWM controlling an electric motor drive means 7 to generate an electric motor controlling voltage V0 for quickly converging the deviation signal xcex94I to zero.
The electric motor drive means 7 is composed of, for example, a bridge circuit using four switching elements such as power FETs (Field Effect Transistors) and each of paired power FETs ( every two FETs constitute one pair) is controlled and driven with an electric motor controlling voltage V0 composed of a PWM control signal and a switch, ON/OFF signal fed from the drive controlling means 6 so that the voltage value and the polarity of the electric motor voltage V to be fed to the electric motor 3. Incidentally, the polarity of the electric motor voltage V is determined by controlling each of the two pairs of power FETs at the electric motor voltage V0 outputted from the drive controlling means 6.
Thus, in the conventional electric power steering apparatus, the feedback loop of the electric motor detecting current I is provided to thereby control the deviation signal xcex94I between the target current Im and the electric motor detecting current I. Therefore, in either case where the steering direction is in the clockwise direction or in the counterclockwise direction, the deviation signal xcex94 is converged into zero so that the electric motor current I quickly becomes the target current Im.
Also, the steering torque sensor of the conventional electric power steering apparatus is known which is composed of a potentiometer and a differential amplifier. The steering torque is detected in terms of a corresponding potential change of the potentiometer. This change is converted into an electric signal and is detected as the steering torque signal.
FIG. 8 shows a specific circuit structure of the steering torque detecting means 1 shown in FIG. 6. The steering torque detecting means 1 is composed of a potentiometer 1a connected between a power source terminal Er and the ground, a differential amplifier 1b whose non-inverted input terminal is connected to a slide terminal of this potentiometer 1a, a resistor 1c connected between an output terminal of this differential amplifier 1b and the inverted input terminal, and partial voltage resistors 1d and 1e connected between a power source terminal Eo and the ground. The connection point between the partial voltage resistors 1d and 1e is connected to the inverted input terminal of the differential amplifier 1b. 
The steering torque generated by operating the steering wheel (not shown) is detected in terms of the potential change of the potentiometer 1a and the output corresponding to this potential change is generated. The output of the potentiometer 1a is fed to the non-inverted input terminal of the differential amplifier 1b and is compared with a voltage (2.5V) that is half the power source voltage (for example, 5V) from the power source terminal Eo which has been applied to the inverted input terminal. The difference therebetween is amplified and outputted as the steering torque signal Ta.
FIG. 9 shows characteristics between the steering torque T and the steering torque signal Ta.
In FIG. 9, according to the characteristics the steering torque signal actually shows (indicated by the solid line), the steering torque signal Ta detected by the steering torque detecting means 1 and corresponding to the steering torque T=0 is 2.5V and is increased in a linear fashion from 2.5V to 5V as the steering torque T is increased in a plus (+) direction (for example, in the clockwise direction), whereas the steering torque signal Ta is decreased in a linear fashion from 2.5 V to 0 V as the steering torque T is increased in a minus (xe2x88x92) direction (for example, in the counterclockwise direction).
Accordingly, the steering torque detecting means 1 is so constructed that the torque reference value is set to 2.5 V and the magnitude and the direction of the steering torque is detected as the steering torque signal Ta of from 0 V to 5 V.
Also, FIG. 9 shows an example of the characteristics, indicated by the one-dot-and-dash line, in which the steering torque signal Ta of the torque reference value detected by the steering torque detecting means 1 and corresponding to the steering torque T=0 is offset in a Lower direction from the voltage of 2.5 V. This means that the steering torque T is offset by xcex94T in the minus direction (xe2x88x92) (for example, in the counterclockwise direction). When the steering torque T is increased by xcex94T in the plus (+) direction (for example, in the clockwise direction), the steering torque signal Ta is detected at 2.5 V.
Incidentally, in the adjustment of the torque reference value of the steering torque detecting means 1, in assembling the electric power steering apparatus, for example, the resistance (variable resistances) of one of the partial resistors 1d and 1e connected to the inverted input terminal of the differential amplifier 1b is adjusted to set the steering torque signal Ta to 2.5 V as the initial setting.
By the way, the above-described conventional power steering apparatus suffers from the following problems.
Namely, the adjustment of the torque reference value of the steering torque detecting means is performed only in assembling work, and thereafter the centering adjustment is not performed. Accordingly, the torque reference value would be changed due to the voltage offset: of the potentiometer that constitutes the steering torque detecting means and the aging change of the differential amplifier, or due to the shift of the adjustable resistance (variable resistance) caused by vibrations or the like, resulting in imbalance in detection of the steering torque. Thus, there is a fear that a driver would feel unsmoothness in steering operation.
Also, depending on the setting of the target value Im and the steering torque T as shown in FIG. 7, there is a fear that the electric motor current might continue to flow while the steering operation is not effected due to the offset of the torque reference value. Accordingly, a non-sensitive zone (neutral zone) xcex94Tf is provided as indicated by the broken line. The neutral zone xcex94Tf has to be broader than the value range in which the torque reference value would be offset. Since this is significantly broad, there is a problem that the operationability would be deteriorated.
The present invention has been made to solve the above-noted problems, and an object of the present invention is therefore to provide an electric power steering apparatus that is superior in operationability and reliability and is capable of keeping stable steering feeling by automatically compensating the torque reference value even if the characteristics of the steering torque detecting means change.
According to a first aspect of the present invention, there is provided an electric power steering apparatus comprising: a steering torque detecting means for detecting a steering torque to be applied to a steering system of a vehicle; a vehicle velocity detecting means for detecting a vehicle velocity of the vehicle; a curvature radius calculation means for calculating a radius of curvature of a road along which the vehicle travels; a torque reference value storage means for storing a torque reference value of the steering torque; a torque compensation means for compensating the torque reference value of the steering torque on the basis of the steering torque detected by the steering torque detecting means in response to the vehicle velocity detected by the vehicle velocity detecting means and a radius of curvature detected by the curvature radius calculation means; a compensation controlling means for calculating an inferential steering torque on the basis of the torque reference value obtained by the torque compensation means the steering torque detected by the steering torque detecting means; and a target current setting means for setting a target current on the basis of the vehicle velocity detected by the vehicle velocity detecting means and the inferential steering torque.
According to a second aspect of the present invention, in the electric power steering apparatus of the first aspect of the invention, the torque compensation means detects an offset of the torque reference value from: a steering torque obtained by the steering torque detecting means in the case where the vehicle travels along a curve either on the right or left with any desired radius of curvature obtained by the curvature radius calculating means and at any desired vehicle velocity obtained by the velocity detecting means; a steering torque obtained by the steering torque detecting means in the case where the vehicle travels along a curve in the opposite direction to that of the above-described curve with the same radius of curvature as the above-described radius of curvature and at the same velocity as the above-described vehicle velocity; and the differences between the torque reference value and the former steering torque and the differences between the torque reference value and the latter steering torque, to thereby determine a torque compensation value for compensating the torque reference value of the steering torque.
According to a third aspect of the present invention, in the electric power steering apparatus of the second aspect of the invention, the torque compensation means comprises a torque signal storage means for storing the steering torque signal detected by the steering torque detecting means and a torque compensation value setting means for outputting a torque signal reception command to the torque signal storage means when the radius of curvature detected by the curvature radius calculation means and a vehicle velocity detected by the vehicle velocity detecting means are any desired values and for setting a new torque reference value on the basis of the torque reference value from the torque reference storage means and a steering torque signal fed back at that time.
According to a fourth aspect of the present invention, in the electric power steering apparatus of the third aspect of the invention, the torque compensation value setting means comprises a torque compensation allowance judgment means and a torque signal compensation means, the torque compensation allowance judgment means outputting a torque signal reception command to the torque signal storage means when the radius of curvature from the curvature radius calculation means and the vehicle velocity from the vehicle velocity detecting means are any desired values, and sending the steering torque signal fed back at that time to the torque signal compensation means, and the torque signal compensation means receiving the steering torque signals sent from the torque compensation allowance judgment means when the vehicle travels along the right curve and the left curve, comparing the difference between the torque reference value and the right and left steering torque signal, and storing a value obtained by adding half that value to the above torque reference value as a new torque reference value.
According to a fifth aspect of the present invention, in the electric power steering apparatus of the third aspect of the invention, the torque compensation value setting means comprises a storage means and a torque signal compensation means, the storage means storiing the steering torque signals detected by the steering torque detecting means for each representative value of the radius of curvature obtained by the curvature radius calculation means and for each representative value of the vehicle velocity detected by the vehicle velocity detecting means, and the torque signal compensation means extracting only a portion in which the steering torque signals on the right and left sides of the same curvature radius and the same vehicle velocity from the storage means and compensating the torque reference value according to an average value of all errors between the steering torque signals and the torque reference value.
According to a sixth aspect of the invention, the electric power steering apparatus of the first aspect of the invention further comprises a torque reference value determining means for compensating the torque reference value, obtained by the torque compensation means, only when the vehicle velocity detected by the vehicle velocity detecting means is lower than a desired value.